1. Field of the Invention
The present invention relates to flow sensing elements for a flow sensor that measures the volume of suction air and the like in an internal combustion engine, and especially to thermal type flow sensing elements each having a diaphragm structure where a thin-film sensor portion is formed on a substrate, and a method of manufacturing the same.
2. Description of the Background Art
In conventional methods of manufacturing thermal type flow sensing elements each having a diaphragm structure, after thin-film sensor portions are formed of a thermal resistance film on a substrate surface, regions of the substrate below the thin-film sensor portions are removed to form diaphragms. The substrate is then cut and separated into individual flow sensing elements. These separated flow sensing elements are incorporated into a flow sensor after processing.
For example, Japanese Patent Application Laid-open No. 10-206205 (1998) discloses one example of a method of manufacturing thermal type flow sensing elements each having a thin-film sensor portion formed on a substrate surface.
The thin-film sensor portion of each of the above flow sensing elements includes a heat generating resistance having a resistance value that varies with high heat generated by energization; thus, the characteristics of the heat generating resistance can change from the initial state. Accordingly, it is necessary to stabilize the characteristics by providing electrical connection between the flow sensing elements and the exterior and by energizing the flow sensing elements so as to previously bring their respective heat generating resistances to a predetermined temperature or more.
An example of the techniques for providing electrical connection as above described is a technique for bringing the electrodes of the flow sensing elements into direct contact with pins. This case, however, presents the problem of possible damage to the electrodes, thereby resulting in degraded reliability.
To avoid such degradation in reliability, there is a technique for supplying energization after assembling a flow sensor into a condition to readily establish connection with the exterior. In this case, however, an increase in the size of a structure leads to an increase in the size of facilities for energization, thereby resulting in a decrease in workability.
Further, in the processing of the above separated flow sensing elements, a protective film is formed on the thin-film sensor portions of a thermal resistance film. If this protective film has defects in coverage due to cracks, foreign material and the like, electrolysis may occur at these defects in coverage when the flow sensing elements are exposed to water. At this time, since changes in the resistance values cause variations in the characteristics from the initial state, the resistances may become open in the worst case. It is therefore necessary to check whether or not defects due to cracks, foreign material and the like extend to the thermal resistance film, but it is difficult to observe the flow sensing elements in sections. Hence, as for defects due to foreign material, manufacturing is controlled such that the flow sensing elements are directly observed from the surface side by eye or by a microscope, etc., for the size of foreign material, and if the size exceeds a predetermined value, the foreign material is judged to be a defect. However, depending on the shape of foreign material, even where the foreign material appears small from direct observation from the surface, it can actually be a large defect. Therefore, it is difficult to accurately detect all defects due to foreign material from direct observation from the surface side by eye or by a microscope, etc. The same can be said of defects due to cracks. Accordingly, inspection by direct observation from the surface has the problem of possible degradation in reliability.
To avoid such degradation in reliability, there is a technique for performing inspection by, instead of conducting direct observations, energizing the flow sensing elements with their bodies immersed in water and observing, by eye or by a microscope, etc., the generation of air bubbles due to electrolysis at defects in coverage. In this case, however, energization is required for each of a plurality of flow sensing elements, and thus, there are other problems of complicated manufacturing process steps and increased manufacturing costs.